1. Field of the Invention
The present invention relates to a lapping device for polishing a surface of a magnetic drum used in a magnetographic printing machine. In particular, it relates to an automatic lapping device used for obtaining a required surface finish of the magnetic drum during the production of the magnetographic printing machine, and for removing a layer of deposited matter such as toner or paper dust formed on the surface thereof after the magnetographic printing machine has been operated for a certain period.
2. Description of the Related Art
In the printing process of the magnetographic printing machine, a magnetic latent image formed on the surface of the magnetic drum is converted to a visual image by adhering toner particles to the surface of the magnetic drum, and the toner particles on the drum surface are then transferred to and fixed on a surface of a paper, to provide a hard copy of the image.
Consequently, after a certain period of operation, a thin film-like layer of deposited plastic component contained in the toner particles is formed on the magnetic drum surface. The formation of this thin film is called "filming". Also, a layer is formed on the drum surface by the depositing thereon of foreign matter such as paper dust.
To form the latent image on the magnetic drum, the drum surface is scanned by a magnetic recording head, and during this scanning, a carriage supporting the magnetic recording head is moved in parallel to the axis of rotation of the magnetic drum by a vertical scanning motor. (This operation is called "vertical scanning".) During this vertical scanning, the magnetic drum is rotated at a high speed by a recording motor, and the magnetic recording head is controlled in accordance with the phase angle of the drum rotation and the position of the magnetic recording head during the vertical scanning, to magnetize a required area of the drum surface.
The magnetic recording head is supported at a predetermined distance from the drum surface during the vertical scanning by an air bearing formed by an air stream caused by the rotation of the rotating drum. If filming occurs on the drum surface, the distance between the magnetic recording head and the drum surface to be magnetized becomes larger than the predetermined distance, and therefore, it becomes difficult to obtain a clear hard copy because the magnetizing intensity of the drum surface is lowered due to this larger distance.
Moreover, if the magnetic recording head comes into contact with the deposits of foreign matter on the drum surface, during the vertical scanning, the magnetic recording head rebounds away from the drum surface, and accordingly, the drum surface is not magnetized in the area at which this rebounding occurs and blank lines appear in the printed images.
To prevent an occurrence of the above defects, the drum surface must be periodically polished to remove foreign matter therefrom. Usually, this polishing of the drum surface (hereinafter called "lapping") is carried out by pressing a lapping sheet by hand against the surface of the drum while the drum is rotated at a high speed. The lapping sheet is usually a plastic film having an abrasive material such as aluminum oxide coated on the surface thereof.
Nevertheless, a problem arises with regard to this polishing or lapping by hand, in that it is difficult to press the lapping sheet against the drum surface with a constant force, and thus it is very difficult for even a skilled person to obtain a uniform lapping of the entire surface of the drum. Further, in this lapping operation since the lapping sheet must be pressed by hand against a drum surface rotating at a high speed, this is not desirable from the view point of safety.
An object of the present invention is to solve the above-described problems and to provide an automatic lapping device by which the surface of the drum is automatically lapped in a safer manner, and a uniform lapping of the drum surface is ensured.